/**
 * https://ac.nowcoder.com/acm/problem/14706
 */

#include<iostream>
using namespace std;
int main()
{
    int h,c,p;
    cin>>h>>c>>p;
    int t;
    t=c/p;
    int v=h/t;
    if(h%t)
        v++;
    if(v>10)
        cout<<"-1"<<endl;
    else
        cout<<v<<endl;
    return 0;
}